Monitoring the ship traffic increasing throughout the world is based today predominantly on radar monitoring, voice radio and the utilization of AIS (Automatic Identification System). Since the year 2000, the AIS has been specified as obligatory standard by the International Maritime Organization (IMO) in order to increase the safety of international ship traffic. This locally limited radio system is used for exchanging navigations and other ship data which are intended to enable the ships to obtain a comprehensive overview of the adjacent ship traffic. The primary aim in this context is to avoid collision between ships.
With the aid of an AIS ship transceiver arranged on the ships, AIS radio signals which the corresponding ship data such as, for example, navigation data, are emitted in a broadcast process alternately on two channels in the FM maritime radio band, namely on 161.975 MHz on the one hand, and on 162.025 MHz, on the other hand. Emitting the individual AIS radio signals in this case occurs at fixed time intervals, the occupancy of which is tuned independently by the relevant subscribers of AIS radio cell, so-called SOTDMA (Self Organizing Time Division Multiple Access). Thus, only 2,250 time slots are available per minute to the individual subscribers for a transmission of data.
With the aid of the AIS ship transceiver installed on board, the radio signals emitted by the other ships can be received, wherein the ship data contained in the AIS radio signals can then be extracted. By means of the reception of the ship data of different ships in the environment of a ship, an overview of the adjacent ship traffic or of the adjacent ships can thus be derived so that a ship can find out, for example, in what direction and with what speed adjacent ships are traveling which, for example, are not directly visible due to poor sight conditions. This makes it possible therefore to avoid collisions and to generate a significantly improved, even if locally limited, situational overview.
Due to the FM frequency band used, the radio range of AIS from ship to ship corresponds to approximately 40 to 60 km, which corresponds to slightly more than the normal visual range on the high sea. Due to their higher positions, coast stations can cover a circle of up to 100 km. Due to the limited range and the SOTDMA transmission protocol used, ships which can see and receive one another form an AIS radio cell within which the subscribers can transmit and receive without collision.
The AIS thus only represents a local radio system which, although it provides sufficient time slot capacity for a ship on the high sea, is not suitable however for a worldwide, especially satellite-based survey of the ship traffic since the coverage of a satellite comprises many individual AIS radio cells. However, a real-time survey of the AIS ship traffic data occurring worldwide would be of great interest to shipping companies, the maritime organizations or environmental ministries, especially in order also to counteract illegal machinations on the high sea.
Thus, a global ship monitoring system for example is known from the post-published DE 10 2011 113 152 A1 by means of which ships of ship traffic on the high sea can be monitored centrally. For this purpose, commercial aircraft have AIS receiving units which receive the AIS radio signals emitted by the ships along their flight route and transmit them for further processing to a base station. By this means, an almost gapless coverage and monitoring can be achieved especially on much-traveled routes.
In the recent past, trials have been undertaken to arrange AIS receiving antennas on satellites in order to be able to receive the AIS radio signals emitted worldwide with the aid of a satellite system. Thus, a satellite-based system is known, for example, from US 2008/0086267 A1 in which an AIS receiving unit is arranged at the satellites in order to be able to receive the AIS radio signals emitted by the ships. The AIS information contained in the AIS radio signals is forwarded to a central station in order thus to obtain global monitoring of the ship traffic. By means of such a satellite system, gapless monitoring thus can also be achieved in remote regions.
The Western industrial nations, in particular, demand the introduction of control of the marine traffic from space as part of their anti-terror campaign. With regard to AIS radio signals in highly frequented maritime regions, however, such satellite-based monitoring systems have the considerable disadvantage that the detection of ships can no longer be implemented with sufficient accuracy. Due to the extreme altitude of a satellite, an area of reception having a diameter of approx. 5000 to 6000 km is covered. Since the AIS as a local radio system organizes itself independently into individual radio cells, all of which transmit on the same frequency band, such a large radius of reception however leads to the reception of a multiplicity of radio cells having identical transmitting frequency and time slots so that AIS radio signals of different AIS radio cells become superimposed and normal data processing is no longer possible. It is especially on highly frequented shipping routes or in the vicinity of harbors and coasts that the area of reception of the satellite covers such a large number of ships that sufficiently reliable monitoring is no longer possible due to the signal collisions.
To counter this problem, two additional FM channels are provided from now on for satellite-based monitoring with the aid of AIS radio signals, in addition to the AIS frequencies already existing (AIS1: 161.975 MHz; AIS2: 162.025 MHz). For this purpose, for the communication, the channels 156.775 MHz (AIS3) and 156.875 (AIS4) are to be allocated especially for the reception of AIS radio signals from space by satellite. The background to introducing the additional satellite AIS (SAT-AIS) frequencies is to obtain as complete as possible a maritime situational overview with the aid of SAT-AIS on the high sea away from the coastal regions. However, it is not currently possible with AIS1 and AIS2 for the above-mentioned reasons (interferences).
According to the ITU definition (ITU-R-M-1371-4), AIS3 and AIS4 radio signals are switched off automatically at the ship when the ships are within the communication range of so-called AIS base stations which generally are installed at the coast. If a ship receives a “base station report” (message 4), sent out by an AIS base station with the demand to switch off SAT-AIS (control bit for “transmission control for long range broadcast message”), the AIS ship transceiver installed on the ship deactivates the emission of the AIS3/AIS4 radio signals for the satellite-based monitoring. This is intended to achieve in coastal regions which, as a rule, are highly traveled due to the concentration of ship traffic, no AIS3/AIS4 radio signals should be emitted in order not to disturb the satellite reception due to a multiplicity of radio signals, which can become mutually superimposed. Monitoring is then done via the AIS base stations. If the ship is located outside the communication range of an AIS base station, the emission of the AIS3/AIS4 radio signals is switched on again automatically for satellite-based monitoring at the ship.
However, the automatic switching-off function has the disadvantage that this functionality can be misused. It is thus conceivable, for example, that pirates or other terrorist organizations force the AIS ship transceiver to be switched off by means of a fake AIS base station message although the ship is not in range of an AIS base station. In the worst case, this would lead to the ship not emitting any AIS radio signals for satellite-based monitoring and the ship can thus no longer be monitored and located. However, it is also conceivable that by forging such an AIS base station message, the crew of a ship themselves switch off the AIS ship transceiver for satellite-based monitoring and thus, for example, perform illegal activities or escape access by security authorities.